Summer Moon Illusion

June 20, 2005: Sometimes you can't believe your eyes. This
week is one of those times.

Step outside any evening at sunset and look around. You'll see a
giant moon rising in the east. It looks like Earth's moon, round and
cratered; the Man in the Moon is in his usual place. But something's
wrong. This full moon is strangely inflated. It's huge!

You've just experienced the Moon Illusion.

Sky watchers have known this for thousands of years: moons hanging
low in the sky look unnaturally big. Cameras don't see it, but our
eyes do. It's a real illusion.

Above:
A time-lapse sequence of the moon rising over Seattle. To the camera,
the moon appears to be the same size no matter what its location on
the sky. Credit and copyright: Shay Stephens. [More]

This week's full moon hangs lower in the sky than any full moon since
June 1987, so the Moon Illusion is going to be extra strong.

What makes the moon so low? It's summer. Remember, the sun and the
full Moon are on opposite sides of the sky. During summer the sun
is high, which means the full moon must be low. This weekâs
full moon occurs on June 22nd, barely a day after the summer solstice
on June 21st--perfect timing for the Moon Illusion.

When you look at the moon, rays of moonlight converge and form an
image about 0.15 mm wide in the back of your eye. High moons and low
moons make the same sized spot. So why does your brain think one is
bigger than the other? After all these years, scientists still aren't
sure why.

A
similar illusion was discovered in 1913 by Mario Ponzo, who drew two
identical bars across a pair of converging lines, like the railroad
tracks pictured right. The upper yellow bar looks wider because it
spans a greater apparent distance between the rails. This is the "Ponzo
Illusion."

Some researchers believe that the Moon Illusion is Ponzo's
Illusion, with trees and houses playing the role of Ponzo's converging
lines. Foreground objects trick your brain into thinking the moon
is bigger than it really is.

But there's a problem. Airline pilots flying at very high altitudes
sometimes experience the Moon Illusion without any objects in the
foreground. What tricks their eyes?

Maybe it's the shape of the sky. Humans perceive the sky as a flattened
dome, with the zenith nearby and the horizon far away. It makes sense:
Birds flying overhead are closer than birds on the horizon.
When the moon is near the horizon, your brain, trained by watching
birds, miscalculates the moon's true distance and size.

There are other explanations,
too. It doesn't matter which is correct, though, if all you want to
do is see a big beautiful moon. The best time to look is around moonrise,
when the moon is peeking through trees and houses or over mountain
ridges, doing its best to trick you. The table below (scroll down)
lists moonrise times for selected US cities.

A fun activity: Look at the moon directly and then through a narrow
opening of some kind. For example, 'pinch' the moon between your thumb
and forefinger or view it through a cardboard tube, which hides the
foreground terrain. Can you make the optical illusion vanish?

Stop that! You won't want to miss the Moon Illusion.

Moonrise over Selected
US Cities

City

Time
Zone

June
20

June
21

June
22

New
York, NY

EDT

7:25 p.m.

8:39 p.m.

9:43 p.m.

San
Diego, CA

PDT

6:58 p.m.

8:09 p.m.

9:14 p.m.

Washington,
DC

EDT

7:31 p.m.

8:44 p.m.

9:49 p.m.

Honolulu,
HI

HST

6:17 p.m.

7:26 p.m.

8:31 p.m.

Chicago,
IL

CDT

7:28 p.m.

8:42 p.m.

9:46 p.m.

Houston,
TX

CDT

7:18 p.m.

8:28 p.m.

9:34 p.m.

Denver,
CO

MDT

7:32 p.m.

8:45 p.m.

9:48 p.m.

Miami,
FL

EDT

7:03 p.m.

8:13 p.m.

9:19 p.m.

Seattle,
WA

PDT

8:22 p.m.

9:38 p.m.

10:39 p.m.

If your city does not appear in
the list, click
here for more data
from the US Naval Observatory.

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